DESCRIPTION: This project investigates the neuroendocrine mechanisms by which mammals generate seasonal variations in physiological and behavioral traits in synchrony with changing environmental conditions. Prior studies have examined responses of Siberian, Phodopus sungorus, and Syrian hamsters, Mesocricetus auratus, to static long and short day lengths of early summer and winter, respectively. Proposed experiments characterize how these responses differ as a function of prior day length exposure. Accumulating evidence establishes that the normal functioning of photoperiodic systems can only be accurately assessed by incorporating the incrementally and continuously changing pattern of day lengths under which photoperiodic time measurement systems evolved. Accordingly, the proposed experiments make extensive use of naturalistic progressions of day lengths and assess changes in body weight and gonadal condition. Additional experiments simulate directly the melatonin patterns associated with intermediate and gradually changing day lengths in animals with neurological lesions and other manipulations of circadian systems. Specific experiments address 1) whether the gradual decrease in body weight in autumn reflects graded responses to progressively decreasing day lengths or the output of an internally timed mechanism triggered by a single critical day length; 2) how the speed at which the endogenous interval timer runs is influenced by the day lengths experienced early in development; 3) how past melatonin exposures, reproductive status and entrainment state of the circadian system contribute to the interpretation of current melatonin signals; 4) whether prior day lengths influence production of future melatonin signals or responses to them; and 5) whether circadian systems are involved in discriminating inhibitory from stimulatory melatonin signals. The proposed experiments are relevant to an understanding of human seasonal rhythms in cardiovascular disease, neurodevelopmental disorders, immune function, sleep duration,, body weight, depression and non-psychiatric mood states. This basic research will add to a body of literature contributing to the development and testing of rational therapies for mood and sleep disorders, jet lag, and shift work. Finally, these studies provide basic information about how seasonal mechanisms may respond to unnatural changes in melatonin signals such as may occur in humans self-administering oral dosages of melatonin.